Walk into bike shop X, and even if they sell 6k double squish bikes with 700 dollar forks, chances are if you get a new shift cable and housing installed, you’ll walk away with cheesy plastic housing ferrules. These ferrules exist because the universe seeks balance. Plastic ferrules have to be really bad, so that really good brass machined ferrules can exist. If the plastic crap did not exist, the universe would spin off its axis and crash into a Walmart. Plastic ferrules are flexy (bad for good shifting), cheap looking, and prone to exploding, cracking, or bending.
If you still use cable operated brakes (and you should!), it’s likely that the housing for those brakes use chrome plated steel ferrules. These are sorta better than plastic, but they’re prone to rusting, bending, splitting, and blowing up too. They’re terrible, but again, 99% of all bikes have them. Having either plastic or steel ferrules on your bike is like driving around a Ferrari with 87 octane gas. Why? To save a few bucks when the rest of the car cost 250k? Brass Ferrules cost money, it’s true. They’re really expensive for such a tiny little piece of metal. But here’s the deal. They are stiff and strong. I’ve never bent or crimped one, or seen a split, exploded, dented, or wallowed out one. They’re so well made that when you recable the bike, you can reuse the ferrules. They’re brass colored, so they look cool, and tarnish nicely. If the cable rubs on them, brass has a basic ‘self lubricating’ quality, which just means that it’s slick enough to use in things like brake bushings, or pedal bushings.
Get em in 5mm or 4mm, we stock the 4mm 'cause some bikes only take 4mm housing, but if you have the option and are recabling your bike you should get some 5mm Jagwire Housing.